Steam-operated power plants, especially coupled gas and steam power plants, have a water-steam cycle which sometimes can also be designed as one or more assisted circulation steam generators with steam drums and also associated heating surfaces. Such assisted circulation steam generators are commonly divided into a high-pressure stage, an intermediate pressure stage and a low-pressure stage depending on its working pressure regime. Generated inside individual pressure stages of the water-steam cycle, by absorbing thermal energy, is water steam (simply referred to as steam in the following text) which can be fed to one or more steam turbines for electric power generation. Instead of being designed as an assisted circulation steam generator of the power plant, the steam generator can also be designed as a forced flow steam generator (Benson boiler, Sulzer boiler, etc.). Such assisted circulation steam generators, however, are for the most part provided only in the high-pressure stage of the water-steam cycle, but can basically also be provided for the lower pressure stages.
On account of numerous chemical and physical processes, waste water, which is provided to a greater or lesser extent with impurities, accumulates in the water-steam cycle during operation of the power plant. So as not to impair the readiness of the power plant by these impurities during operation, it is necessary to drain the power plant and therefore to remove the impurities or the waste water from the water-steam cycle. Such draining is typically undertaken during continuous operation of the power plant. During this, the drainage waters are discharged from lines, which are normally closed during normal operation, in which the waste water has collected. For the discharging process, the subject lines are opened for a short time and the drainage waters are drained away. During the draining, water is therefore lost to the water-steam cycle and, renewed by make-up water, so-called deionized water, has to be fed to the water-steam cycle.
In addition, condensation water, which opposes an efficient utilization of the water-steam cycle, also collects in the lines of said water-steam cycle. Such condensation water is formed especially on account of time-related changing operating conditions in the water-steam cycle. Condensation water thus accumulates in the water-steam cycle when power plants are shutting down, for example, since with reducing operating temperatures the steam which is present in the water-steam cycle increasingly condenses out and the thereby accumulating condensed water also collects in parts of the plant which are not intended for a longer contact with liquid water. In this respect, it is necessary when a power plant is shutting down to increasingly remove water from the water-steam cycle in order to avoid undesirable condensation of water in parts of the plant which are not intended for it. At the same time, less water is replenished into the water-steam cycle when shutting down in order to keep relevant parts of the plant largely free of condensed water at the end of the shutting down process.
In order to remove such condensation water from the water-steam cycle, suitable drainage lines, which are fluidically connected to the water-steam cycle, are also put into service. Sometimes, these are identical to the drainage lines for draining impure waste waters from the water-steam cycle.
At this point, reference is to be made to the fact that in the sense of the present invention, drainage waters of the power plant can be both impure waste water, such as sludge, and condensed water which has collected in regions of the water-steam cycle which are not intended for it.
According to the prior art, it is already known to collect and to conduct together drainage waters from different parts of the water-steam cycle, especially from different pressure stages. In this case, the drainage waters, as described for example in WO 2006/058845 (US 2008 0104959 A1), or in US 2007 0289304 A1, can be temporarily stored in a tank for further treatment.
It is disadvantageous to these solutions of the prior art, however, that by temporarily storing the drainage waters, the thermal energy contained therein cannot be further utilized. Rather, when ejecting the drainage waters the thermal energy is discharged from the power plant into the environment without being utilized. Moreover, it proves to be disadvantageous that the make-up water which is introduced into the water-steam cycle for replacing the discharged drainage waters have to be thermally treated again in order to be raised to a temperature level which corresponds or comes sufficiently close to the water already present in the water-steam cycle. This in turn requires expenditure of thermal energy and disadvantageously disrupts the energy balance of the power plant. Moreover, it proves to be disadvantageous that the drainage waters which are discharged from the water-steam cycle have to be treated in an energetically costly process in order to separate especially the sludge from water which is reusable as deionized water. It proves to be especially unfavorable with regard to resource balance if the treated water is no longer fed back into the water-steam cycle but for example is ejected into the environment.
According to these disadvantages which are known from the prior art it proves to be technically necessary to propose a solution for draining a power plant which avoids the disadvantages which are known from the prior art. The technical solution which is to be proposed is especially to enable an energetically advantageous utilization of the energy which is extracted as a result of discharging drainage waters from the water-steam cycle. In other words, draining which is improved with regard to the overall energy balance of the power plant operation is to be undertaken. Furthermore, it is desirable to re-utilize the energy discharged from the water-steam cycle and also the drainage waters for the power plant and especially for the water-steam cycle.